Friday, January 11, 2019

Metallurgy in the Americas – Part V

Continued from the previous post regarding the presence of metallurgy in South America, where archaeologists claim metallurgy began, and from there traveled northward into Central, Meso-, and North America. It was also discussed that Andean metallurgists used alloys almost exclusively to mold their images.
    In Ecuador, metallurgical activity was present from at least 1500 BC, with regional styles early evolving and giving rise to high levels of technical craftsmanship and to shaping particular iconographic and decorative patterns. Copper, gold and silver were mined, processed and converted into thousands of ornaments, offerings, tools and weapons extensively used both by elites and by the common people (Roberto Lleras Perez, Metallurgy in Ancient Ecuador (Ministry of Culture of Ecuador), Archaeopress Archaeology, Oxford, England, 2015).
Dotted Circle: Central Andes; Red Circle: Northern Highlands; Blue Circle: Altiplano; where major metallurgical sites operated anciently

In the central Andes, two major metallurgical traditions emerged and exchanged influences. One centered in the northern highlands and the adjacent coast, where copper, arsenic, gold and silver ores were abundant; the other center was the Altiplano with its oxide and sulfide copper ores and tin as well as silver and gold (Karen Olsen Bruhns, “Ancient South America,” Cambridge World Archaeology Series, Cambridge University Press, 1994). As these Peruvian works spread northward, we find that in Colombia, where surface metal resources were less abundant, metallurgy arose somewhat later in 500 BC to 200 BC. In these regions metal was handled differently, most often by shaping hollow solid forms by casting them in molds by the "lost-wax technique” (Dorothy Hosler, “The Sounds and Colors of Power: The Sacred Metallurgical Technology of Ancient West Mexico,” MIT Press, 1994).
    In contrast to the abundance in Peru, in Bolivia there is a paucity of both natural alloys and artifacts made of arsenic bronze. Rather, tin bronze and copper–arsenic–nickel alloys seem to have been the metals utilized. According to Lechtman, the earliest occurrence of this alloy in Bolivia is found on the Bolivian Altiplano before 600 AD, with tin bronze favored for ornamental rings, while copper–arsenic– nickel appears to have been preferred for needles, nails, and chisels. The tin for tin bronze appears to have been obtained from the rich “tin belt” of the Altiplano, where it primarily occurs in the mineral cassiterite (tin oxide).
    No source has yet been found for the copper–arsenic–nickel alloys, with the tin-based bronze metallurgy in northern Bolivia beginning before 600 AD. This is broadly contemporaneous with the widespread use of arsenic bronze metallurgy in nearby southern Peru and northwest Argentina. Future research is needed to understand what, if any, interaction was occurring between Bolivia and southern Peru at this time of florescence of the copper industry.
The Valley of Túcume contains 26 pyramids covering 544 acres and one of the most important archaeological sites in Peru, located near the coastal city of Chiclayo (Lambayeque (Sicán) and Chimú cultures

Traveling north, the Sicán culture site of Batán Grande represents early arsenic bronze metallurgy along the north coast of Peru. Previously undocumented in the New World, Batán Grande is a prehistoric metallurgical center situated in the La Leche Valley, where smelting began early and continued until just prior to the Spanish conquest. Though small quantities of copper ore are locally available, arsenic bearing minerals are not. However, the highlands of northern Peru are rich in arsenic bearing minerals—a fact leading metallurgists to believe that highland miners might have provided coastal smelters with the necessary arsenic.
    In addition, although artifacts made of bronze alloys are the most commonly found during archaeological excavation, by Inca times, gold and silver remained the most prized metals, However, artifacts composed purely of silver or gold are extremely rare, with alloys containing a mixture of copper and gold or silver, were considerably more common. Even after the Spanish invasion, such was the case and the conquistadors were surprised that in melting down their confiscated Peruvian “golden objects” were in fact composed of copper. For centuries during pre-Columbian times, Peruvian smiths developed techniques of combining copper interiors of object in sophisticated alloying techniques had golden surfaces on alloys containing small percentages of precious metals.
    Experimental archaeology has been especially important here in determining how Andean cultures manipulated alloys to accentuate desired qualities. Two of the best examples of native abilities were the processes of electrochemical replacement plating and depletion gilding.
    In “electrochemical replacement,” a copper alloy was given an extremely thin and even surface coating of silver or gold. To accomplish this, silver and gold were dissolved in an acidic or corrosive solution, then a copper artifact was dipped into this solution, and a chemical reaction would occur that resulted in a very thin and even “plate” or surface coating of silver or gold. In addition, the specific color of the object could be altered simply by varying the relative amount of silver or gold in the solution.
Depleted gilding on an Andean pectoral ornament, first century AD

The other technique, “depletion gilding” was used on alloys of copper, silver, and gold, with naturally-occurring chemicals used to separate the gold from the silver, leaving a surface of the desired precious metal. This gilding is a method for producing a layer of nearly pure gold on an object made of gold alloy by removing the other metals from its surface. It is sometimes referred to as a "surface enrichment" process. 
    Ancient objects made of noble metal alloys, that is, gold with copper and/or silver, can show the phenomenon of surface enrichment. This phenomenon is regarding the composition of the surface, which has a percentage of gold higher than that of the bulk. This enrichment, called gilding, covers several techniques for applying a gold leaf or a gold powder to solid surfaces, in order to have a thin coating of this metal on objects. 
    In this method, a layer of nearly pure gold is placed on an object made of gold alloy by removing the other metals from its surface in this "surface enrichment" process (Eleanor Susan Blakelock, "Never Judge a Gold Object by its Surface Analysis: A Study of Surface Phenomena in a Selection of Gold Objects,” Archaeometry, vol.58, Iss.6, 2015, pp912–929). This depletion gilding process was used by pre-Columbian populations for their “tumbaga,” a gold-copper alloy, to give it the luster of gold (Amelia Carolina Sparavigna, “Depletion Gilding: An Ancient Method for Surface Enrichment of Gold Alloys,” Mechanics, Materials Science & Engineering, Department of Applied Science and Technology, Polytechnic University of Turin, Italy, January 2016).
    These are just two of the techniques in which native South Americans manipulated the appearance of metal artifacts in order to achieve a surface of silver or gold. These technologies appear to have been developed by the Moche in the first century AD on the north coast of Peru and remained a northern phenomenon until the rise of the Inca Empire in the mid-fifteenth century.
    In short, a wide variety of metallurgical techniques were used by Andean cultures, and considerable skill was demonstrated in the manipulation of nonferrous ores from the last few centuries BC into the first several centuries AD. By integrating the fields of archaeology, ethno-history, and geology a great deal can still be learned about these cultures’ use of metals.
(See the next post, “Metallurgy in the Americas – Part VI,” regarding the fact that almost all metallurgists claim that metallurgy developed and began in South America and from there traveled northward into Central, Meso-, and North America)

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